Why is AsH3 trigonal planar and OF2 linear in VSEPR theory?

[kasse]

Homework Statement

What is the geometry of AsH₃ according to the VSEPR model?

The Attempt at a Solution

As has 5 valence electrons and H has one. The molecule therefore has four sets of electron pairs, whereof one is unshared. This gives a tetrahedral structure where the As-H angles are 107 degrees.

However, according to the solutions manual the geometry is triangular planar. Why?


I also have a problem with the geometry of OF₂. Here, there are four sets of electron pairs, whereof two are unshared. I would expect the geometry of this molecule to be bent, but it's linear according to my book.

 

[MichaelXY]

For the OF2, I think it is due to the formal charges. O and F are both negative, where in the case of H2O the O is neg and the H is +. This is my first year of chem, so I may be wrong.

 

[eli64]

1. As has 5 valence electrons and H has one. The molecule therefore has four sets of electron pairs, whereof one is unshared. This gives a tetrahedral structure where the As-H angles are 107 degrees.

However, according to the solutions manual the geometry is triangular planar. Why?


I also have a problem with the geometry of OF₂. Here, there are four sets of electron pairs, whereof two are unshared. I would expect the geometry of this molecule to be bent, but it's linear according to my book.

For AsH3 you are on the right track - keep thinking about the shape - but your solution manual is wrong

For OF2 you are correct and the manual is wrong again

 

[kasse]

For AsH3 you are on the right track - keep thinking about the shape

Hm, I couldn't figure it out.

 

[eli64]

shape depends on atoms

 

[MichaelXY]

Eli is correct. I am sorry for my misinformation.
http://en.wikipedia.org/wiki/Oxygen_difluoride

By the way, What book are you using?

 

[eli64]

Homework Statement

What is the geometry of AsH₃ according to the VSEPR model?

The Attempt at a Solution

As has 5 valence electrons and H has one. The molecule therefore has four sets of electron pairs, whereof one is unshared. This gives a tetrahedral structure where the As-H angles are 107 degrees.

yes electrons are in the tetrahedral arrangement and dictate the arrangement of the outside atoms. so what shape has 4 pairs of electrons, 1 unshared and 3 outside atoms?

 

[elemis]

Sorry to be digging up old threads but is the answer Trigonal Pyramidal for AsH3 ?

 

[nobahar]

I can't say with absolute certainty; but yes, I would conclude that AsH₃ is trigonal pyramidal, like Ammonia. As it has three bonds to Hydrogen, and a lone pair.

 

[k-rod AP 2010]

Yes AsH3 is pyramidal, as the lone pair wants as much space as it can get, and the largest angle it could have from the other atoms would be on top of the molecule. leaving the other hydrogen atoms in a trigonal pyramidal shape.

http://upload.wikimedia.org/wikipedia/commons/d/da/AX3E1-3D-balls.png
shows what it would look like, w/ the yellow atom being the lone pair.